U.S. patent number 10,254,368 [Application Number 15/548,691] was granted by the patent office on 2019-04-09 for magnetic resonance imaging that nulls signals from two or more tissues to better delineate an interested tissue.
This patent grant is currently assigned to Beth Israel Deaconess Medical Center, Inc.. The grantee listed for this patent is BETH ISRAEL DEACONESS MEDICAL CENTER. Invention is credited to Tamer Basha, Reza Nezafat.
![](/patent/grant/10254368/US10254368-20190409-D00000.png)
![](/patent/grant/10254368/US10254368-20190409-D00001.png)
![](/patent/grant/10254368/US10254368-20190409-D00002.png)
![](/patent/grant/10254368/US10254368-20190409-D00003.png)
![](/patent/grant/10254368/US10254368-20190409-D00004.png)
![](/patent/grant/10254368/US10254368-20190409-M00001.png)
United States Patent |
10,254,368 |
Basha , et al. |
April 9, 2019 |
Magnetic resonance imaging that nulls signals from two or more
tissues to better delineate an interested tissue
Abstract
A system and method for acquiring magnetic resonance imaging
(MRI) images with an MRI system is provided. The system and method
directs the MRI system first to produce an inversion recovery radio
frequency (RF) pulse, wait for a time period, produce a
T2-preparation RF pulse, wait for another time period, and then
acquire data of a part of a subject. The first produced RF pulse
rotates net magnetization 180 degrees about an axis. The pulse
sequence used to acquire data can be any two-dimensional or
three-dimensional sequence used to acquire a volume in the subject.
The two waiting time periods are chosen such that the signals of
two or more tissues of the subject are nulled.
Inventors: |
Basha; Tamer (Revere, MA),
Nezafat; Reza (Waban, MA) |
Applicant: |
Name |
City |
State |
Country |
Type |
BETH ISRAEL DEACONESS MEDICAL CENTER |
Boston |
MA |
US |
|
|
Assignee: |
Beth Israel Deaconess Medical
Center, Inc. (Boston, MA)
|
Family
ID: |
56564577 |
Appl.
No.: |
15/548,691 |
Filed: |
February 2, 2016 |
PCT
Filed: |
February 02, 2016 |
PCT No.: |
PCT/US2016/016097 |
371(c)(1),(2),(4) Date: |
August 03, 2017 |
PCT
Pub. No.: |
WO2016/126659 |
PCT
Pub. Date: |
August 11, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180031660 A1 |
Feb 1, 2018 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
62112903 |
Feb 6, 2015 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B
5/055 (20130101); G01R 33/483 (20130101); G01R
33/5602 (20130101); A61B 2576/023 (20130101) |
Current International
Class: |
A61B
5/055 (20060101); G01R 33/56 (20060101); G01R
33/483 (20060101) |
Field of
Search: |
;324/309,322
;600/413 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report and Written Opinion for International
Patent Application No. PCT/US2016/016097 dated Jun. 27, 2016. cited
by applicant .
Xie et al., "3D Flow-Independent Peripheral Vessel Wall Imaging
Using T2-Prepared Phase-Sensitive Inversion-Recovery Steady-State
Free Precession." Journal of Magnetic Resonance Imaging 32:399-408
(2010), pp. 400-401 [online] <URL:
http://onlinelibrary.wiley.com/doi/10.1002/jmri.22272/epdf>.
cited by applicant.
|
Primary Examiner: Hoque; Farhana A
Attorney, Agent or Firm: Quarles & Brady LLP
Government Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
This invention was made with government support under EB008743
awarded by the National Institutes of Health. The government has
certain rights in the invention.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application represents the U.S. National Stage of
International Application No. PCT/US2016/016097, filed Feb. 2, 2016
which claims the benefit of U.S. Provisional Patent Application
Ser. No. 62/112,903, filed on Feb. 6, 2015, and entitled "System
and Method for Magnetic Resonance Imaging That Nulls Signals from
Two or More Tissues to Better Delineate an Interested Tissue."
Claims
The invention claimed is:
1. A method for using a magnetic resonance imaging (MRI) system to
produce an image of a subject, the steps of the method comprising:
(a) producing with the MRI system, an inversion recovery (IR) radio
frequency (RF) pulse; (b) waiting a first delay time following the
IR RF pulse during which magnetic resonance signals from a first
tissue, a second tissue, and a third tissue in a subject positioned
in the MRI system evolve according to respective longitudinal
relaxation times for the first, second, and third tissues; (c)
producing with the MRI system after the first delay time, a
T2-preparation RF pulse having a duration; (d) waiting a second
delay time following the T2-preparation RF pulse during which
magnetic resonance signals from the first, second, and third
tissues in the subject evolve according to respective transverse
relaxation times for the first, second, and third tissues; (e)
acquiring data from the subject with the MRI system during a data
acquisition window following the second delay time, wherein the
first delay time, the duration of the T2-preparation RF pulse, and
the second delay time are selected such that magnetic resonance
signals from the first tissue and the second tissue are nulled
during the data acquisition window; (f) reconstructing an image of
the subject from the acquired data, wherein the reconstructed image
depicts magnetic resonance signals from the third tissue and not
from the first and second tissues.
2. The method as recited in claim 1, further comprising: generating
a contrast map for each of the first, second, and third tissues,
each contrast map depicting magnetic resonance image contrast
values for respective ones of the first, second, and third tissues
as a function of different values for the first delay time, the
duration of the T2-preparation RF pulse, and the second delay time;
and wherein steps (a)-(c) include selecting the first delay time,
the duration of the T2-preparation RF pulse, and the second delay
time based on the generated contrast maps for the first, second,
and third tissues.
3. The method as recited in claim 2, wherein generating the
contrast maps for the first, second, and third tissues includes
acquiring data from the subject with the MRI system over a range of
different values for the first delay time, the duration of the
T2-preparation RF pulse, and the second delay time.
4. The method as recited in claim 2, wherein generating the
contrast maps for the first, second, and third tissues includes
generating the contrast maps using a numerical simulation based on
magnetic resonance signal models for each of the first, second, and
third tissues.
5. The method as recited in claim 2, wherein the first delay time,
the duration of the T2-preparation RF pulse, and the second delay
time are further selected to maximize magnetic resonance signals
from the third tissue during the data acquisition window while
magnetic resonance signals from the first tissue and the second
tissue are nulled during the data acquisition window.
6. The method as recited in claim 1, wherein the first tissue is
blood, the second tissue is myocardium, and the third tissue is
infarcted myocardium.
7. The method as recited in claim 1, wherein the T2-preparation RF
pulse is a composite RF pulse.
8. The method as recited in claim 7, wherein the composite RF pulse
includes a 90 degree RF pulse followed by a train of 180 degree RF
pulses followed by a -90 degree RF pulse.
9. A method for generating an image of a subject with a magnetic
resonance imaging (MRI) system, the steps of the method comprising:
(a) providing to a computer system: a first contrast map having
pixel values associated with magnetic resonance image contrast for
a first tissue as a function of a first timing parameter that
defines a duration of time between an inversion recovery (IR) radio
frequency (RF) pulse and a T2-preparation RF pulse and a second
timing parameter that defines a duration of time between the
T2-preparation RF pulse and a data acquisition window; a second
contrast map having pixel values associated with magnetic resonance
image contrast for a second tissue as a function of the first
timing parameter and the second timing parameter; and a third
contrast map having pixel values associated with magnetic resonance
image contrast for a third tissue as a function of the first timing
parameter and the second timing parameter; (b) analyzing the first,
second, and third contrast maps with the computer system to
determine a combination of the first timing parameter and second
timing parameter that will result in maximum magnetic resonance
signal from the third tissue while magnetic resonance signals from
the first tissue and second tissue are simultaneously nulled; (c)
communicating the combination of the first timing parameter and the
second timing parameter to an MRI system; (d) acquiring data with
the MRI system using a pulse sequence that includes an IR RF pulse,
a T2-preparation pulse, and a data acquisition window spaced apart
in time according to the combination of the first timing parameter
and the second timing parameter; and (e) reconstructing an image of
the subject from the acquired data, wherein the image of the
subject depicts the third tissue without signal contributions from
the first and second tissues.
10. The method as recited in claim 9, wherein the first, second,
and third contrast maps are provided by performing numerical
simulations that estimate magnetic resonance image contrast at a
plurality of different first and second timing parameters.
11. The method as recited in claim 9, wherein: the first, second,
and third contrast maps are three-dimensional maps that have pixel
values associated with magnetic resonance image contrast for the
first, second, and third tissues, respectively, as a function of
the first timing parameter, the second timing parameter, and a
third timing parameter that defines a duration of the
T2-preparation RF pulse; and step (b) includes analyzing the first,
second, and third contrast maps with the computer system to
determine a combination of the first, second, and third timing
parameters.
12. The method as recited in claim 9, wherein step (b) includes
identifying the combination of the first and second timing
parameters by identifying a common pixel location in the contrast
map for the first tissue and the contrast map for the second tissue
at which magnetic resonance signals from the first and second
tissues are both nulled.
13. The method as recited in claim 12, wherein step (b) includes
identifying a plurality of combinations of the first and second
timing parameters at which magnetic resonance signals from the
first and second tissues are both nulled and then identifying the
combination of the first and second timing parameters from this
plurality of combinations that maximizes magnetic resonance signals
from the third tissue.
14. The method as recited in claim 9, wherein the first tissue is
blood, the second tissue is myocardium, and the third tissue is
infarcted myocardium.
15. A method for determining an optimal set of timing parameters
for a magnetic resonance pulse sequence implementing an inversion
recovery radio frequency (RF) pulse and a T2-preparation RF pulse,
the steps of the method comprising: (a) providing to a computer
system: a first contrast map having pixel values associated with
magnetic resonance image contrast for a first tissue as a function
of a first timing parameter that defines a duration of time between
an inversion recovery (IR) radio frequency (RF) pulse and a
T2-preparation RF pulse, a second timing parameter that defines a
duration of time between the T2-preparation RF pulse and a data
acquisition window, and a third timing parameter that defines a
duration of the T2-preparation RF pulse; a second contrast map
having pixel values associated with magnetic resonance image
contrast for a second tissue as a function of the first, second,
and third timing parameters; and a third contrast map having pixel
values associated with magnetic resonance image contrast for a
third tissue as a function of the first, second, and third timing
parameters; (b) analyzing the first, second, and third contrast
maps with the computer system to determine a set of the first,
second, and third timing parameters that will result in maximum
magnetic resonance signal from the third tissue while magnetic
resonance signals from the first tissue and second tissue are
simultaneously nulled; and (c) storing the set of the first,
second, and third timing parameters as instructions to be provided
to a magnetic resonance imaging (MRI) system.
16. The method as recited in claim 15, wherein step (b) includes
searching a parameter space defined by the first, second, and third
timing parameters for each of the first, second, and third tissues
to determine the combination of the first, second, and third timing
parameters.
17. The method as recited in claim 15, wherein the first tissue is
blood, the second tissue is myocardium, and the third tissue is
infarcted myocardium.
Description
BACKGROUND OF THE INVENTION
Magnetic resonance imaging ("MRI") is used to noninvasively assess
the function of heart with good image quality and without risk of
radiation. One major cardiac application is infarct imaging,
imaging of infarcted tissue--scar--after a heart attack. Because
scars can be small and with unknown shapes, it is desirable to
control the MRI system so that the contrast between scar and all
other major tissues in the heart in the generated images is
maximized to a level that scar can be discernible from other major
tissues.
It would therefore be highly desirable to provide a system and
method for magnetic resonance imaging ("MRI") in which the contrast
between scar and all other major tissues is maximized.
SUMMARY OF THE INVENTION
The present invention overcomes the aforementioned drawbacks by
providing a method for using a magnetic resonance imaging (MRI)
system to produce an image of a subject. The MRI system is operated
to produce an inversion recovery (IR) radio frequency (RF) pulse
and to wait a first delay time following the IR RF pulse. During
this first delay time, magnetic resonance signals from a first
tissue, a second tissue, and a third tissue in a subject positioned
in the MRI system evolve according to respective longitudinal
relaxation times for the first, second, and third tissues. The MRI
system is then operated after the first delay time to produce a
T2-preparation RF pulse having a duration and to wait a second
delay time following the T2-preparation RF pulse. During this
second delay time, magnetic resonance signals from the first,
second, and third tissues in the subject evolve according to
respective transverse relaxation times for the first, second, and
third tissues. Data are then acquired from the subject with the MRI
system during a data acquisition window following the second delay
time. The first delay time, the duration of the T2-preparation RF
pulse, and the second delay time are selected such that magnetic
resonance signals from the first tissue and the second tissue are
nulled during the data acquisition window. An image of the subject
is then reconstructed from the acquired data. This reconstructed
image depicts magnetic resonance signals from the third tissue and
not from the first and second tissues.
It is another aspect of the invention to provide a method for
generating an image of a subject with a magnetic resonance imaging
(MRI) system. The method includes providing first, second, and
third contrast maps to a computer system. The first, second, and
third contrast maps have pixel values associated with magnetic
resonance image contrast for a first, second, and third tissue,
respectively, as a function of a first timing parameter that
defines a duration of time between an IR RF pulse and a
T2-preparation RF pulse and a second timing parameter that defines
a duration of time between the T2-preparation RF pulse and a data
acquisition window. The first, second, and third contrast maps are
analyzed with the computer system to determine a combination of the
first and second timing parameters that will result in maximum
magnetic resonance signal from the third tissue while magnetic
resonance signals from the first tissue and second tissue are
simultaneously nulled. This combination of the first and second
timing parameters is then communicated to an MRI system. The MRI
system is operated to acquire data using a pulse sequence that
includes an IR RF pulse, a T2-preparation pulse, and a data
acquisition window spaced apart in time according to the
combination of the first timing parameter and the second timing
parameter. An image of the subject is reconstructed from the
acquired data. This reconstructed image of the subject depicts the
third tissue without signal contributions from the first and second
tissues.
It is another aspect of the invention to provide a method for
determining an optimal set of timing parameters for a magnetic
resonance pulse sequence implementing an inversion recovery (IR)
radio frequency (RF) pulse and a T2-preparation RF pulse. The
method includes a first, second, and third contrast map to a
computer system. The first, second, and third contrast maps have
pixel values associated with magnetic resonance image contrast for
a first, second, and third tissue, respectively, as a function of a
first timing parameter that defines a duration of time between an
IR RF pulse and a T2-preparation RF pulse, a second timing
parameter that defines a duration of time between the
T2-preparation RF pulse and a data acquisition window, and a third
timing parameter that defines a duration of the T2-preparation RF
pulse. The first, second, and third contrast maps are analyzed with
the computer system to determine a set of the first, second, and
third timing parameters that will result in maximum magnetic
resonance signal from the third tissue while magnetic resonance
signals from the first tissue and second tissue are simultaneously
nulled. This set of the first, second, and third timing parameters
is then stored as instructions to be provided to a magnetic
resonance imaging (MRI) system.
The foregoing and other aspects and advantages of the invention
will appear from the following description. In the description,
reference is made to the accompanying drawings which form a part
hereof, and in which there is shown by way of illustration a
preferred embodiment of the invention. Such embodiment does not
necessarily represent the full scope of the invention, however, and
reference is made therefore to the claims and herein for
interpreting the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an example plot of signal intensities of tissues at
various times after an inversion pulse when a standard late
gadolinium enhancement (LGE) method is used.
FIG. 2 is an example of a pulse sequence that may be used to
implement the system and method as disclosed herein.
FIG. 3 is an example plot of signal intensities of tissues at
various times after an inversion pulse when a system and method as
disclosed herein is used.
FIG. 4 is a example flowchart that may be used to simulate the
effects of .DELTA.t.sub.1, .DELTA.t.sub.2, and .DELTA.t.sub.3 on
signal intensities of tissues.
FIG. 5A depicts example contrast maps at various .DELTA.t.sub.1 and
.DELTA.t.sub.3 values for different tissue types using a preset
T.sub.2-preparation pulse duration.
FIG. 5B depicts tissue nulling lines derived from the contrast maps
of FIG. 5A and that indicate a combination of .DELTA.t.sub.1 and
.DELTA.t.sub.3 values that will simultaneously null two particular
tissue types at a nulling time point.
FIG. 6 is a block diagram of an example of a magnetic resonance
imaging ("MRI") system.
DETAILED DESCRIPTION OF THE INVENTION
Described here are systems and methods for magnetic resonance
imaging ("MRI") using a pulse sequence that is designed such that
signals originating from two or more particular tissue types are
simultaneously nulled. The pulse sequence is preferably designed
such that this simultaneous nulling is achieved while also
maximizing image contrast for a desired tissue type. In this
manner, the tissue-of-interest can be better delineated in the
acquired magnetic resonance images.
As one non-limiting example, the pulse sequence can be designed to
use an optimized combination of an inversion pulse and a
T.sub.2-preparation composite pulse to simultaneously null both
healthy myocardium and blood signals, thereby producing a so-called
black-blood ("BB") image without losing significant contrast
between infarcted and healthy myocardial tissue. In general, this
result is achieved using a pulse sequence that includes an
inversion recovery pulse and a T.sub.2-preparation pulse that are
optimally timed and designed to simultaneously null the blood and
healthy myocardium signals during a data acquisition window, while
also maximizing the achievable signal from scar tissues.
For instance, a first delay time between the inversion pulse and
the T.sub.2-preparation pulse, the duration of the
T.sub.2-preparation pulse, and a second delay time between the
T.sub.2-preparation pulse and a data acquisition window are all
selected such that signals from two particular tissue types (e.g.,
blood and healthy myocardial tissue in one example) are nulled
during the data acquisition window. As will be described below,
these timing parameters can be determined or estimated using
numerical simulations, or by acquiring patient-specific data using
a quick scouting sequence that samples the parameter space from
which the optimal timing parameters can be determined. An example
of an MRI system that can implement these methods is described
below with respect to FIG. 6.
As described above, MRI can be used to image and detect scar tissue
in an infarcted heart. Three major types of tissues are present in
a infarcted heart: blood, healthy myocardium, and scar tissue. To
discern scar tissue from its surroundings, contrast between scar
and all other tissues should be increased to a desirable level. For
example, it is desirable if scar tissue can be made to appear
significantly brighter in an image than blood and healthy
myocardium. Such contrast would allow scar tissue to be assessed
with confidence.
Late gadolinium enhancement ("LGE") imaging is a standard MRI
method for infarct imaging. LGE can be used to depict scar and
fibrosis in patients with cardiovascular diseases. In LGE, a
gadolinium-containing contrast agent is administered to a subject,
such as via an intravenous injection. The contrast agent diffuses
rapidly out of capillaries into tissue, but cannot cross intact
cell membranes. After the intravenous bolus, both healthy
myocardium and scar tissue passively accumulate contrast agent.
But, with time the scar tissue will possess a slightly larger
amount of contrast agent per unit volume of tissue because of the
contrast agent slower kinetics and larger volume for
distribution.
The MRI pulse sequence typically used in LGE often starts with an
inversion recovery ("IR") radio frequency ("RF") pulse.
Gadolinium-containing contrast agents are T.sub.1-shortening
agents, which means that tissues in the presence of the agents will
experience shorter T.sub.1 than the tissues not near the agents.
Referring to FIG. 1, the normalized net magnetization of three
different tissues (scar, blood, and healthy myocardium) at various
times after the end of the IR pulse is plotted. The magnetization
recovery curves for these tissues are illustrated as line 102 for
scar tissue, line 104 for blood, and line 106 for healthy
myocardium. Immediately after the IR pulse is applied, the net
magnetization is -M.sub.0. Afterwards, the magnetization recovers
back to M.sub.0 according to an exponential function with a time
constant T.sub.1. As described above, because of having a larger
volume of contrast agents per unit, scar (line 102) has shorter
T.sub.1 than myocardium (line 106). The acquisition time of LGE is
often chosen at the nulling point of myocardium so that myocardium
appears black and scar appears bright. This way, a contrast
difference between myocardium and scar based on T.sub.1 is
created.
But, blood (line 104) also appears bright in the images acquired
with standard LGE. In fact, blood and scar have similar T.sub.1
values, so the contrast between blood and scar is not sufficient to
differentiate the two tissues. Referring still to FIG. 1, the blood
signal is around 75 percent of the scar signal at the myocardium
null point 108 (i.e., the contrast between blood and scar tissue is
low). This low contrast makes sub-endocardial scar challenging to
depict and detect, especially in thin structures such as right
ventricle or left atrium. Other methods have been used in attempt
to increase the blood-scar contrast, but suffer from either reduced
SNR or reduced scar-myocardium contrast. Moreover, most of these
techniques are not compatible with three-dimensional ("3D")
acquisitions, which have recently emerged to completely cover the
heart in viability imaging.
The systems and methods described here combine an inversion pulse
and a T.sub.2-preparation composite pulse that are optimally timed
and designed to simultaneously null two tissue types, while
maximizing contrast with a third tissue type. As one non-limiting
example, the pulse sequence can be designed to simultaneously null
signals from blood and healthy myocardium, while maximizing the
signal intensity achievable in scar tissue, and thus significantly
increasing the image contrast between scar tissue and blood and
healthy myocardium. In general, the T.sub.2-preparation pulse is
inserted between the inversion pulse and the data acquisition and
the temporal spacing between the two RF pulses and the data
acquisition, in addition to the duration of the T.sub.2-preparation
pulse, are designed to achieve the results described above.
FIG. 2 illustrates an example of a pulse sequence that implements
the techniques described above. The pulse sequence includes
applying an inversion pulse 202 before applying a
T.sub.2-preparation pulse 204 and implementing a data acquisition
206. The inversion pulse 202 may be accompanied with a
slice-selective or slab-selective gradient, or may be a
non-slice-selective or non-slab-selective RF pulse. After a time
period, .DELTA.t.sub.1, the T.sub.2-preparation 204 is applied. The
T.sub.2-preparation lasts for a duration of time, .DELTA.t.sub.2.
The T.sub.2-preparation pulse 204 may be a composite
T.sub.2-preparation pulse sequence that includes a series of RF
pulses starting with a 90 degree RF pulse, followed by a train of
180 degree RF pulses, and ending with a -90 degree RF pulse. In
this example, the train of 180 degree RF pulses are used to remove
T*.sub.2 effects.
When applying a T.sub.2-preparation pulse, the signals from the
tissues also evolve according to an exponential function described
by a different time constant, the transverse relaxation time,
T.sub.2. FIG. 3 illustrates the magnetization recovery curves for
three different tissues (scar tissue, 302; blood, 304; and healthy
myocardium, 306) during the application of an inversion pulse and
T.sub.2-preparation pulse according to the methods described above.
Blood (line 304) has shorter T.sub.1 than healthy myocardium (line
306), so during the time period, .DELTA.t.sub.1, after the
inversion pulse but before the T.sub.2-preparation pulse, blood
(line 304) recovers faster than healthy myocardium (line 306). But,
blood (line 304) has a longer T.sub.2 than healthy myocardium (line
306), so when a T.sub.2'' preparation pulse is applied during the
time period, .DELTA.t.sub.2, the blood signal (line 304) evolves
slower than the healthy myocardium signal (line 306), thereby
reversing the faster recovery that occurred during the first time
period, .DELTA.t.sub.1. After the T.sub.2-preparation pulses are
turned off, the magnetization once again begins recovering to the
longitudinal axis and continues the inversion recovery according to
the longitudinal relaxation time, T.sub.1, of the tissues. With the
adjustment of the time periods .DELTA.t.sub.1, .DELTA.t.sub.2, and
.DELTA.t.sub.3, both blood (line 304) and healthy myocardium (line
306) signals can be nulled at a nulling time point 308 at which the
signals from scar tissue (line 302) are not nulled. Data
acquisition can thus occur at this nulling time point 308.
Preferably, the timing parameters (.DELTA.t.sub.1, .DELTA.t.sub.2,
.DELTA.t.sub.3) are selected to both simultaneously null the
signals from blood and healthy myocardium, while maximizing the
achievable signal from scar tissue. In this manner, the acquired
images will depict blood and healthy myocardium as black, and scar
tissue as bright. As a result, scar tissue can be more readily
differentiated from the other tissues in the heart. By changing the
time periods .DELTA.t.sub.1, .DELTA.t.sub.2, .DELTA.t.sub.3, one
can control the contrast between different tissues in the
heart.
Referring again to FIG. 2, after the T.sub.2-preparation pulses
end, the MRI system waits for a time period .DELTA.t.sub.3 so that
signals from two or more tissues are simultaneously nulled at a
nulling time point. Data is acquired during a data acquisition
window 206 that is timed to occur while signals from these two or
more tissues (e.g., blood and healthy myocardium) are
simultaneously nulled, as mentioned above. Data acquisition can be
performed using any suitable data acquisition sequence, including
any two-dimensional or three-dimensional pulse sequences used to
acquire images of a volume in a subject.
As described above, numerical simulations can be performed using
the Bloch equation to determine or estimate values for
.DELTA.t.sub.1, .DELTA.t.sub.2, and .DELTA.t.sub.3. Referring now
to FIG. 4, an flowchart is illustrated as setting forth the steps
of an example method for estimating these timing parameters. The
method begins by providing T.sub.1 and T.sub.2 values for a
specific tissue-of-interest, in addition to an initial value for
the timing parameter, .DELTA.t.sub.2, which is the duration of the
T.sub.2-preparation pulse, as indicated at step 402. For this
tissue-of-interest, the steps shown generally at 404 are performed
next. First, a set of values for .DELTA.t.sub.1 and .DELTA.t.sub.3
are provided, as indicated at step 406, and then signal intensities
for the tissue-of-interest are calculated using a combination of
values for .DELTA.t.sub.1 and .DELTA.t.sub.3 from this provided set
and using the provided .DELTA.t.sub.2 value, as indicated at step
408. This process 404 is then repeated for all combinations of
.DELTA.t.sub.1 and .DELTA.t.sub.3 within the set of values, which
may span preset ranges for each timing parameter.
When a determination is made at decision block 410 that signal
intensities have been estimated for all desired combinations of
.DELTA.t.sub.1 and .DELTA.t.sub.3 within the set of values, a
determination is made at decision block 412 whether signal
intensities have been estimated for all of the desired
tissues-of-interest. If not, the method loops back to step 402,
where T.sub.1 and T.sub.2 values for a different
tissue-of-interest, in addition to an initial value for the timing
parameter, .DELTA.t.sub.2, are provided. The process at 404 is then
repeated for each desired tissue-of-interest until the signal
intensities for all desired tissues-of-interest have been
estimated, or otherwise simulated, for all desired combinations of
.DELTA.t.sub.1 and .DELTA.t.sub.3 within the set of values. When
the signal intensities for all desired tissues-of-interest have
been estimated, or otherwise simulated, for all desired
combinations of .DELTA.t.sub.1 and .DELTA.t.sub.3 within the set of
values, contrast maps are generated to depict the contrast for a
particular tissue type given a combination of time period
parameters, .DELTA.t.sub.1 and .DELTA.t.sub.3, for each
tissue-of-interest.
In some embodiments, step 402 and process 404 are also repeated for
multiple different T.sub.2-preparation pulse durations,
.DELTA.t.sub.2, within a desired range of such values. In these
embodiments, the contrast maps generated in step 414 are
three-dimensional maps of contrast at various combinations of
.DELTA.t.sub.1, .DELTA.t.sub.2 and .DELTA.t.sub.3. In these
instances, the contrast maps provide information about the behavior
of signal intensities, and the resulting image contrast, for
various tissues-of-interest as a function of a three-dimensional
parameter space defined by the timing parameters, .DELTA.t.sub.1,
.DELTA.t.sub.2 and .DELTA.t.sub.3.
As will be described below, the contrast maps generated in the
manner mentioned above provide information about the magnetization
recovery (and thus signal intensity evolution) over a
two-dimensional or three-dimensional parameter space defined by
combinations of the timing parameters .DELTA.t.sub.1,
.DELTA.t.sub.2 and .DELTA.t.sub.3. Thus, these contrast maps can be
efficiently analyzed to identify combinations of timing parameters
that, when implemented in a pulse sequence such as the one
illustrated in FIG. 2, can allow for the acquisition of magnetic
resonance images in which signals are nulled from two particular
tissue types while optimizing the contrast available for a third
tissue type that is of clinical interest. In the examples described
above, this analysis can be performed to identify a combination of
timing parameters that result in signals being nulled from blood
and healthy myocardium while optimizing the available signal
intensity from scar tissue.
Referring now to FIG. 5A, examples of contrast maps generated using
the methods described above are illustrated. The top panels depict
maps generated using a standard LGE pulse sequence (i.e.,
.DELTA.t.sub.2=0), and the bottom panels depict contrast maps
generated when using the methods described here. In this particular
example, the contrast maps are two-dimensional parameter space maps
for a preset T.sub.2-preparation pulse duration, .DELTA.t.sub.2, of
35 ms. In FIG. 5A, the white dashed line in each contrast map
indicates the so-called null signal line, along which the
combination of .DELTA.t.sub.1 and .DELTA.t.sub.3 results in signals
from that particular tissue type being nulled during data
acquisition.
In FIG. 5B, the null signal lines of healthy myocardium and blood
are illustrated. As shown in the top panel of FIG. 5B, when a
standard LGE sequence is used, the null signal lines of healthy
myocardium and blood do not intersect at a common point, thereby
preventing simultaneous nulling of both tissues. But, when the
method described here is used, a common null point can be obtained
(shown in the bottom panel of FIG. 5B). Identifying this
intersection point results in determining the .DELTA.t.sub.1 and
.DELTA.t.sub.3 parameters that will result in simultaneously
nulling the signals from blood and healthy myocardium during the
data acquisition window.
As the value for .DELTA.t.sub.2 is changed, so does the location of
the signal nulling lines. Thus, some combinations of
.DELTA.t.sub.1, .DELTA.t.sub.2, and .DELTA.t.sub.3 may not result
in an intersection between the signal nulling lines for any two
particular tissues (e.g., blood and healthy myocardium). Similarly,
some combinations of .DELTA.t.sub.1, .DELTA.t.sub.2, and
.DELTA.t.sub.3 will result in higher signal intensities from a
third tissue-of-interest (e.g., scar tissue) than others. A search
through the .DELTA.t.sub.1, .DELTA.t.sub.2, and .DELTA.t.sub.3
parameter space can thus determine the optimal set of parameters
that results in simultaneously nulling two particular tissue types
(e.g., blood and healthy myocardial) while also maximizing the
achievable signal from a third tissue-of-interest (e.g., scar
tissue). It will be readily appreciated that the methods described
here can be readily adapted to any combination of tissue types.
That is, contrast maps can be generated for any desirable
combination of tissue types and the timing parameter space for
these tissues can be searched to identify the combination of timing
parameters that results in the desired image contrast for a
particular clinical or research application.
Compared to standard LGE methods, there is a decrease in the scar
signal intensities in the images acquired at the healthy
myocardium-blood common null point with the method described here.
The amount of decrease depends on .DELTA.t.sub.2. Referring back to
FIG. 3, the recovery of scar signals slows down during
.DELTA.t.sub.2. So, shorter .DELTA.t.sub.2 may yield less decrease
in scar signals. But, .DELTA.t.sub.2 should be long enough to allow
recovery of myocardium that surpasses that of blood, such that both
signals can be nulled after the T.sub.1 recovery during
.DELTA.t.sub.3.
In some embodiments, rather than performing or relying upon
previously performed numerical simulations, a quick scouting
sequence may be used to determine the timing parameters for the
nulling point before the entire volume is imaged. For example,
during scouring, the pulse sequence for the system and method as
disclosed herein (shown in FIG. 2) can be repeated for various
.DELTA.t.sub.1, .DELTA.t.sub.2, or .DELTA.t.sub.3 values. This
scouting sequence can be performed over the entire imaging volume,
or a subset thereof, including only a single slice. The scanning
parameters for scouting images with desired tissues nulled and with
desired signal to noise ratios are chosen as scanning parameters
used to image the entire volume with the system and method as
disclosed herein.
In one configuration, the computer program used for simulations as
described above may also be used to determine optimal
.DELTA.t.sub.1, .DELTA.t.sub.2 or .DELTA.t.sub.3 for the nulling
time point of two or more tissues. A quick scan can be used to
measure the T.sub.1 and T.sub.2 for each tissue, or predetermined
values for the desired tissue types can be used. Then these T.sub.1
and T.sub.2 values are inputted to the method in step 402 in FIG.
4. Using the contrast maps generated in step 414 or the signals
computed in step 408, optimal .DELTA.t.sub.1, .DELTA.t.sub.2, or
.DELTA.t.sub.3 values can be generated with signals of two or more
tissues nulled while the signals of the remaining tissues still
haven desirable signal to noise ratios.
Heart and its major tissues--myocardium, blood, and scar--are used
herein as examples to illustrate the present system and method. One
skilled in the art would appreciate that the system and method
disclosed herein may be applied to image other parts of a subject
with desired contrast among tissues. Also, with certain
combinations of .DELTA.t.sub.1, .DELTA.t.sub.2, and .DELTA.t.sub.3,
signals from more than two tissues may be nulled. In addition, for
the purpose of comparison with standard LGE method,
gadolinium-containing contrast agents are described herein. One
skilled in art would appreciate that the system and method
disclosed herein does not require contrast agents to be used during
imaging.
Referring particularly now to FIG. 6, an example of a magnetic
resonance imaging ("MRI") system 600 is illustrated. The MRI system
600 includes an operator workstation 602, which will typically
include a display 604; one or more input devices 606, such as a
keyboard and mouse; and a processor 608. The processor 608 may
include a commercially available programmable machine running a
commercially available operating system. The operator workstation
602 provides the operator interface that enables scan prescriptions
to be entered into the MRI system 600. In general, the operator
workstation 602 may be coupled to four servers: a pulse sequence
server 610; a data acquisition server 612; a data processing server
614; and a data store server 616. The operator workstation 602 and
each server 610, 612, 614, and 616 are connected to communicate
with each other. For example, the servers 610, 612, 614, and 616
may be connected via a communication system 640, which may include
any suitable network connection, whether wired, wireless, or a
combination of both. As an example, the communication system 640
may include both proprietary or dedicated networks, as well as open
networks, such as the internet.
The pulse sequence server 610 functions in response to instructions
downloaded from the operator workstation 602 to operate a gradient
system 618 and a radiofrequency ("RF") system 620. Gradient
waveforms necessary to perform the prescribed scan are produced and
applied to the gradient system 618, which excites gradient coils in
an assembly 622 to produce the magnetic field gradients G.sub.x,
G.sub.y, and G.sub.z used for position encoding magnetic resonance
signals. The gradient coil assembly 622 forms part of a magnet
assembly 624 that includes a polarizing magnet 626 and a whole-body
RF coil 628.
RF waveforms are applied by the RF system 620 to the RF coil 628,
or a separate local coil (not shown in FIG. 6), in order to perform
the prescribed magnetic resonance pulse sequence. Responsive
magnetic resonance signals detected by the RF coil 628, or a
separate local coil (not shown in FIG. 6), are received by the RF
system 620, where they are amplified, demodulated, filtered, and
digitized under direction of commands produced by the pulse
sequence server 610. The RF system 620 includes an RF transmitter
for producing a wide variety of RF pulses used in MRI pulse
sequences. The RF transmitter is responsive to the scan
prescription and direction from the pulse sequence server 610 to
produce RF pulses of the desired frequency, phase, and pulse
amplitude waveform. The generated RF pulses may be applied to the
whole-body RF coil 628 or to one or more local coils or coil arrays
(not shown in FIG. 6).
The RF system 620 also includes one or more RF receiver channels.
Each RF receiver channel includes an RF preamplifier that amplifies
the magnetic resonance signal received by the coil 628 to which it
is connected, and a detector that detects and digitizes the I and Q
quadrature components of the received magnetic resonance signal.
The magnitude of the received magnetic resonance signal may,
therefore, be determined at any sampled point by the square root of
the sum of the squares of the I and Q components: M= {square root
over (I.sup.2+Q.sup.2)} (1);
and the phase of the received magnetic resonance signal may also be
determined according to the following relationship:
.phi..function. ##EQU00001##
The pulse sequence server 610 also optionally receives patient data
from a physiological acquisition controller 630. By way of example,
the physiological acquisition controller 630 may receive signals
from a number of different sensors connected to the patient, such
as electrocardiograph ("ECG") signals from electrodes, or
respiratory signals from a respiratory bellows or other respiratory
monitoring device. Such signals are typically used by the pulse
sequence server 610 to synchronize, or "gate," the performance of
the scan with the subject's heart beat or respiration.
The pulse sequence server 610 also connects to a scan room
interface circuit 632 that receives signals from various sensors
associated with the condition of the patient and the magnet system.
It is also through the scan room interface circuit 632 that a
patient positioning system 634 receives commands, to move the
patient to desired positions during the scan.
The digitized magnetic resonance signal samples produced by the RF
system 620 are received by the data acquisition server 612. The
data acquisition server 612 operates in response to instructions
downloaded from the operator workstation 602 to receive the
real-time magnetic resonance data and provide buffer storage, such
that no data is lost by data overrun. In some scans, the data
acquisition server 612 does little more than pass the acquired
magnetic resonance data to the data processor server 614. However,
in scans that require information derived from acquired magnetic
resonance data to control the further performance of the scan, the
data acquisition server 612 is programmed to produce such
information and convey it to the pulse sequence server 610. For
example, during prescans, magnetic resonance data is acquired and
used to calibrate the pulse sequence performed by the pulse
sequence server 610. As another example, navigator signals may be
acquired and used to adjust the operating parameters of the RF
system 620 or the gradient system 618, or to control the view order
in which k-space is sampled. In still another example, the data
acquisition server 612 may also be employed to process magnetic
resonance signals used to detect the arrival of a contrast agent in
a magnetic resonance angiography ("MRA") scan. By way of example,
the data acquisition server 612 acquires magnetic resonance data
and processes it in real-time to produce information that is used
to control the scan.
The data processing server 614 receives magnetic resonance data
from the data acquisition server 612 and processes it in accordance
with instructions downloaded from the operator workstation 602.
Such processing may, for example, include one or more of the
following: reconstructing two-dimensional or three-dimensional
images by performing a Fourier transformation of raw k-space data;
performing other image reconstruction algorithms, such as iterative
or backprojection reconstruction algorithms; applying filters to
raw k-space data or to reconstructed images; generating functional
magnetic resonance images; calculating motion or flow images; and
so on.
Images reconstructed by the data processing server 614 are conveyed
back to the operator workstation 602 where they are stored.
Real-time images are stored in a data base memory cache (not shown
in FIG. 6), from which they may be output to operator display 602
or a display 636 that is located near the magnet assembly 624 for
use by attending physicians. Batch mode images or selected real
time images are stored in a host database on disc storage 638. When
such images have been reconstructed and transferred to storage, the
data processing server 614 notifies the data store server 616 on
the operator workstation 602. The operator workstation 602 may be
used by an operator to archive the images, produce films, or send
the images via a network to other facilities.
The MRI system 600 may also include one or more networked
workstations 642. By way of example, a networked workstation 642
may include a display 644; one or more input devices 646, such as a
keyboard and mouse; and a processor 648. The networked workstation
642 may be located within the same facility as the operator
workstation 602, or in a different facility, such as a different
healthcare institution or clinic.
The networked workstation 642, whether within the same facility or
in a different facility as the operator workstation 602, may gain
remote access to the data processing server 614 or data store
server 616 via the communication system 640. Accordingly, multiple
networked workstations 642 may have access to the data processing
server 614 and the data store server 616. In this manner, magnetic
resonance data, reconstructed images, or other data may be
exchanged between the data processing server 614 or the data store
server 616 and the networked workstations 642, such that the data
or images may be remotely processed by a networked workstation 642.
This data may be exchanged in any suitable format, such as in
accordance with the transmission control protocol ("TCP"), the
internet protocol ("IP"), or other known or suitable protocols.
The present invention has been described in terms of one or more
preferred embodiments, and it should be appreciated that many
equivalents, alternatives, variations, and modifications, aside
from those expressly stated, are possible and within the scope of
the invention.
* * * * *
References